This invention relates to a process for joining clad metal components together by welding and to a fabricated clad structure formed thereby.
Among the characteristics of steel which contribute to its widespread use in pressure vessels are its strength and availability at reasonable cost, but steel is attacked by many chemicals and cannot be exposed to such chemicals, particularly at elevated temperatures and pressures. To offset this problem, it has been common practice to clad steel with less reactive, yet more expensive, refractory metals, such as tantalum, zirconium and titanium and their alloys. This, however, presents a whole new set of problems.
Some of the so-called refractory metals which are currently used as cladding for steel plate possess relatively high melting temperatures--indeed considerably higher than that of steel--so that a weld made along the cladding will melt the steel beneath the cladding even though the weld does not fully penetrate the cladding. When the steel again solidifies its physical characteristics are changed, and the result is a region of weakness in the steel backing. This problem has been overcome by interposing a layer of copper between the refractory metal cladding and the steel backing. The copper acts as a heat sink, and although it melts when the refractory metal is welded, it distributes the heat over a widespread area, and this prevents the underlying steel from melting.
The typical pressure vessel possesses a generally cylindrical configuration, and often has hemispherical heads. As such, it is fabricated from various components and segments which are welded together. For example, the hemispherical heads each constitute separate components as does the cylindrical side wall which is interposed between them. Often the heads and the side wall are themselves fabricated from a multitude of segments. These components and segments must be welded together in a manner which presents a totally inert surface toward the interior of the vessel.
U.S. Pat. No. 3,443,306 of J. Meyer entitled Method of Joining Clad Material discloses one procedure for welding together clad steel components when the cladding is tantalum which is separated from the steel backing by a copper intermediate layer. More specifically, the tantalum cladding and copper intermediate layer are stripped away from the steel backing at each edge where the joint is to be formed (FIG. 1), and when the two components are brought together, this creates a groove in the otherwise continuous layer of cladding. The groove exposes the steel backing for each component in the region of the joint, and here the two components are welded together along a butt weld in the steel. Next a filler strip of copper is inserted into the groove and either tack welded or continuously welded in place, and then a flat batten strip, which is formed from the same metal as the cladding is placed over the copper filler strip. The width of the batten strip exceeds the width of the groove so that the edges of the batten strip overlie the cladding on the two components, and along these edges the batten strips are welded to the cladding. Again care must be exercised to prevent the weld from fully penetrating the cladding, for any total penetration will draw molten copper into the weld and render it brittle.
While the foregoing procedure seems relatively simple, it is not. First, the batten strip is difficult to center over the groove and filler strip because it completely obscures the groove. Should it not be centered, the prospects of melting the filler strip or the weld metal which secures it are increased, and of course whenever such a melt occurs, copper or some other foreign metal is drawn into the tantalum weld to weaken it. Secondly, it is common practice to tack weld the batten strip in place before the full welds are made along its edges, but the batten strip, being straight in cross section, has a tendency to distort when tack welded. Finally, the copper filler strip and the welds which are used to secure it represent additional material which increases the cost of the whole procedure.